Yeah, the engineers at Apple that handled my case, said that the speakers are supposed to warm-up or something of that nature, but I still find that to be a drag. The pops are very noticeable in a quiet room.
Since I was outside of my 14 day warranty, Apple opted to try repair my iMac. First, they swapped out the speakers, which didn't do anything. Next, the replaced the logic board, which also didn't resolve the issue.
As a last resort, they finally sent me a new 27 inch iMac. The new iMac I received does not have a speaker popping issue whatsoever and I've had it for about a couple months now. Knock on wood...
So, this leads me to believe that some units are affected, some aren't. I have no idea where the speaker popping stems from, as they literally tried to repair everything on my last unit, but my new iMac seems unaffected.
As many have reported in this thread, an entirely new machine / replacement seems to be the only solution, unless you are wiling to deal with the speaker popping, which to be fair, doesn't seem to affect performance in any way, aside from the total annoyance factor.